1. Field of the Invention
This invention relates to laser diodes, and more particularly to nitride based semiconductor laser diodes and methods for fabricating same.
2. Description of the Related Art
A laser is a device that produces a beam of coherent light as a result of stimulated emission. Light beams produced by lasers can have high energy because of their single wavelength, frequency, and coherence. A number of materials are capable of producing a lasing effect and include certain high-purity crystals (such as ruby), semiconductors, certain types of glass, certain gasses including carbon dioxide, helium, argon and neon, and certain plasmas.
More recently there has been increased interest in lasers made of semiconductor materials because they typically have a smaller size, lower cost, and have other related advantages typically associated with semiconductor devices. Semiconductor lasers are similar to other lasers in that the emitted radiation has special and temporal coherence. Like other lasers, semiconductor lasers produce a beam of light that is highly monochromatic (i.e. of narrow bandwidth) and is highly directional. Overall, semiconductor lasers provide very efficient systems that are easily modulated by modulating the current directed across the devices. Additionally, because semiconductor lasers have very short photon lifetimes, they can be used to produce high-frequency modulation.
A known characteristic of laser diodes (and light emitting diodes (LEDs)) is that the frequency of radiation that can be produced by the particular laser diode is related to the bandgap of the particular semiconductor material. Smaller bandgaps produce lower energy, shorter wavelength photons, while wider bandgaps produce higher energy, shorter wavelength photons. One semiconductor material commonly used for lasers is indium gallium aluminum phosphide (InGaAlP), which has a bandgap that is generally dependant upon the mole of atomic fraction of each element present. This material, regardless of the different element atomic fraction, produces only light in red portion of the visible spectrum, i.e., about 600 to 700 nanometers (nm).
Laser diodes that produce shorter wavelengths not only produce different colors of radiation, but offer other advantages. For example, laser diodes, and in particular edge emitting laser diodes, can be used with optical storage and memory devices (e.g. compact disks (CD) digital video disks (DVD), high definition (HD) DVDs, and Blue Ray DVDs). Their shorter wavelength enables the storage and memory devices to hold proportionally more information. For example, an optical storage device storing information using wavelengths of light in the blue spectrum can hold approximately 32 times the amount of information as one using wavelengths of light in the red spectrum, using the same storage space. This has generated interest in Group-III nitride material for use in laser diodes, and in particular gallium nitride for producing light in the blue and ultra violet (UV) frequency spectrums because of its relatively high bandgap (3.36 eV at room temperature). This interest has resulted in developments related to the structure and fabrication of Group-III nitride based laser diodes [For example see U.S. Pat. Nos. 5,592,501 and 5,838,706 to Edmond et al].
Some edge emitting laser diodes are fabricated with a ridge etched formed in the laser diode's top surface, and in some embodiments, the ridge is etched from the top surfaces of the laser diode, down approximately to the laser diode's active region. The ridge provides electrical and optical confinement, as well as index-guiding for the particular wavelength of light generated by the laser diode. This in turn allows for laser diode operation at lower threshold currents and voltages. These ridges, can be relatively thin, with some ridges being 2 μm or less wide.
The improved operating characteristics achieved by formation of these thin ridges come at the cost of more complex fabrication processes, and in particular more difficult and complex ridge contact deposition. The ridge is typically formed through at least some of the laser diode's p-type layers, which can include the p-type cap layer, waveguide cladding layers, and separate confinement heterostructure. The ridge can be etched through all or some of these layers and can be narrow relative to the overall size of the laser diode and can run down most of the length of the laser diode.
Once the fabrication processes are completed to form the ridge, a p-contact layer must be formed on the top of the ridge with typical p-contact layers made from different combination of nickel, gold and platinum (Ni/Au/Pt). The p-contact layer should electrically contact primarily the top of the ridge to avoid shorting to the layers below. One conventional contacting process is known in the art as the “via process” and involves photolithograph and alignment processes that are designed to align with the ridge for p-contact deposition. These processes, however, are complex and difficult to repeat reliably. This is particularly true for narrow ridges such as those with a width of 2 μm or less. Devices where the alignment is not accurate during fabrication can fail, decreasing the overall yield for the manufacturing process.
Another conventional contacting process is known in the industry as “SiO2 liftoff” and involves using a thick SiO2 layer as the etch mask layer. This process also requires photolithography and alignment steps that can be difficult for narrow ridges. This process also involves depositing and etching a number of different layers, with the result being that the process can be costly, time consuming and unreliable.
Another consideration in contacting the ridge is that the top layer of the ridge is typically formed of a p-type material. This material can be unstable and different environment conditions can damage the surface of the layer. It can be desirable to protect this surface from environmental conditions during processing. The “via process” and “SiO2 liftoff” process described above can expose the p-type material to environmental conditions that can result in damage to the material.